Pass -fPIC to native compiles on 32-bit

Pass -fPIC to native compiles on 32-bit

This is apparently a regression from 1.14.0 to 1.15.0. Previously we
passed `-fPIC` to C compilers on i686 targets, but the `gcc` crate
apparently [explicitly] didn't do this. I don't recall why that was
avoided but it was [previously passed by the makefiles][mk] and this
seems to have [caused a regression][regression] in Firefox, so this
commit reverts back to passing `-fPIC`.

[explicitly]: https://github.com/alexcrichton/gcc-rs/commit/362bdf20
[mk]: https://github.com/rust-lang/rust/blob/c781fc4a/mk/cfg/i686-unknown-linux-gnu.mk#L11
[regression]: https://bugzilla.mozilla.org/show_bug.cgi?id=1336155